Researchers from the University of Strathclyde and the University of St. Andrews have demonstrated that ammonia can be synthesized directly from air (instead of N2) and H2O (instead of H2) under a mild condition (room temperature, one atmosphere) with supplied electricity which can be obtained from renewable resources such as solar, wind or marine. In addition to providing a less carbon-intensive pathway for ammonia, their process could also reduce the pressure on renewable energy storage, they note.

Their paper appears in Scientific Reports, the open access journal from the Nature Publishing Group.

Globally 131 million tons of ammonia were produced in 2010. The dominant ammonia production process is the Haber-Bosch process invented in 1904 which requires high temperature (~500°C) and high pressure (150–300 bar), in addition to efficient catalysts. Natural gas or coal is used as the energy source of the ammonia industry. 1.87 tons of CO2 is released per ton of ammonia produced. Globally 245 million tons of CO2 were released by the ammonia industry in 2010 equivalent to about 50% of the UK CO2 emissions (495.8 million tons) in that year. In the Haber-Bosch process, the presence of ppm level oxygen may poison the commonly used Fe-based catalysts. In industry, extensive purification of N2 and H2 is needed and this remarkably increases the overall cost of the process. Therefore researchers have been seeking a simpler way for synthesis of ammonia from nitrogen separated from air.

...It is well known that some higher plants can synthesize ammonia or its derivatives directly from air and water at room temperature. The ammonia produced by plants is normally directly used as fertilizer by the plants. To the best of our knowledge, there is no report on artificial synthesis of ammonia direct from air and water. It has been a dream for researchers who can imitate this natural process to synthesize ammonia under similar conditions.

...In most reports, H2 and N2 were commonly used as precursors for electrochemical synthesis of ammonia while H2 production and N2 separation are essential. H2 production can be bypassed if H2O was used as a precursor; however, the reaction between H2O and N2 to form ammonia is thermodynamically non-spontaneous under normally pressure; however, this can be achieved through electrochemical process because the applied voltage provides extra driving force.

—Lan et al.

In their study, they first fabricated an electrochemical cell for ammonia synthesis. H2 (or water) and N2 (or air) were passed through room temperature water first then filled into the chambers of the cell. The DC potential was applied by a Solartron 1470A electrochemical interface controlled by software for automatic data collection.

A maximum ammonia production rate of 1.14 × 10−5 mol m−2 s−1 was achieved when a voltage of 1.6 V was applied. Potentially, this can provide an alternative route for the mass production of the basic chemical ammonia under mild conditions, they concluded.

Comments

Their production rate equates to ~1 mol (17g) of ammonia per day per square metre. That seems pretty low to me. The useful figure would be to compare energy per mol using the electrochemical approach vs. Haber-Bosch process.

anhydrous ammonia can be used as a fertilizer directly. The rate doesn't have to be high, just continuous and the apparati distributed. Sure, no one will get rich, but sometimes we should do things for the benefit of mankind, not corporate america. Yes, I know, that's crazy talk.

Well there is a thought. If we reach a stage where solar PV supply starts to exceed daytime demand a portion could be diverted to generate ammonia, reducing the need for the carbon intensive Haber-Bosch process. Even if the energy efficiencies were lower for the electrochemical approach, there would still be a net benefit by utilising excess low carbon energy.

I agree that the useful figure would be to compare energy per mol using this approach vs. Haber-Bosch process. Considering ammonia production accounts for a few percent of the world's energy consumption, a more energy efficient route would be a significant development (throughput is also important as mentioned). Reacting hydrogen with nitrogen should in principle at least allow the PRODUCTION of energy, due to the unreactivity of nitrogen in practice a lot of energy is CONSUMED creating the enormous pressures and high temperature for the reaction with a catalyst.

This might lead to something useful in the future, but it probably needs to bump up by a couple orders of magnitude. Let's assume they can match the surface area to volume ratio of a human lung. In order to generate the 131MTons of ammonia they would need the equivalent of 250 million gallons of lungs. Then add in the down time needed to replace the membranes (the paper did not suggest a useable lifespan) which will not last forever.

More advanced crop management techniques would be more useful at reducing the expenditure of energy on ammonia production. However, this technique may very well lead to something 20 times better that could work.

The problem is not that the Haber-Bosch process is carbon-intensive. The problem is that the H2-source is carbon-intensive.
When excess-renewable energy is used to make H2 from water, you can also have carbon-free amonia.

Although there may be other advantages of this new process, also without it we can have completely carbon-free amonia.
(And the production of carbon-free H2 can obviously have other useful application)

There are those that are suggesting NH3 as a alternative "green" fuel. If it's produced this way, they might actually have a point. But since currently it's produced through the Haber-Bosch process, it's anything but green.

My question about this new process is that it's waste byproduct would be oxygen. Not exactly a bad thing, but I can see that it would have the potential to poison the catalyst, since it would oxidize pretty much anything short of platinum or gold.